Bug Fixes

• Fix Subscriber state after releasing messages
  • Correctly reset the Subscriber state when releasing messages
    after the callback either raises an error, or the callback
    fails to call acknowledge or modify_ack_deadline on the
    message. If a Subscriber fills it's inventory, and stops
    pulling additional messages before all the callbacks are
    completed (moves to a paused state) then the Subscriber
    could become stuck in a paused state.
  • A paused Subscriber will now check whether to unpause after
    the callback is completed, instead of when acknowledge or
    modify_ack_deadline is called on the message.